One method for selective adsorptive separation of nitrogen from nitrogen-containing gas is a pressure swing (PSA) method. The PSA method employs an adsorbent such as a zeolite, and achieves adsorptive separation between nitrogen and oxygen by repeating a cycle which comprises the steps of separating oxygen from a nitrogen/oxygen gas mixture such as air by selectively adsorbing highly polar nitrogen on the adsorbent under an increased pressure, desorbing nitrogen from the adsorbent under a reduced pressure, and regenerating the adsorbent.
It is known that the zeolite employed in the PSA method has a polar component adsorption capacity which varies depending on the type of associated cations in the zeolite, and an adsorption amount increases as the charge density of the cations increases (Literature 1: H. Minato and Watanabe, Sci. Pap. Coll. Gen. Educ. Univ. Tokyo 28, p.215-220 (1978)). The number of the cations in the zeolite corresponds to Al in the skeleton of the zeolite. Therefore, a zeolite having a greater number of Al atoms is more advantageous because a greater number of cations serving as adsorption sites are present therein. Further, an adsorption amount and diffusion rates during the adsorption and the desorption are increased, as the diameters and volumes of pores in the crystalline structure of the zeolite are increased.
From this viewpoint, an A-zeolite (MS5A) and an X-zeolite of faujasite structure (CaX) which are ion-exchanged with Ca are conventionally employed as adsorbents for oxygen production PSA on an industrial basis.
It has recently been found that an x-zeolite ion-exchanged with lithium ions, particularly a low-silica X-zeolite (LSX), has a much higher capacity than the conventional MS5A and Cax, and such an X-zeolite has been put into practical use as an adsorbent for oxygen production PSA (Literature 2: U.S. Pat. No. 4,859,217 and Japanese Examined Patent Publication No. HEI5-25527) Further, effective use of divalent cations in combination with the lithium ions for the ion exchange has also been patented (Literature 3: U.S. Pat. No. 5,152,813 and Japanese Examined Patent Publication No. HEI7-57300).
However, a lithium source for the ion exchange is more expensive than other cation components, and the ion exchange is very difficult. More specifically, the Li+-LSX is typically prepared by exchanging monovalent cations (Na+, K+) in a Na+-LSX or a Na+.K+-LSX with lithium ions. As can be understood from an ion-exchange isotherm observed when sodium ions of the Na+-X-zeolite were exchanged with lithium ions (Literature 4: H. S. Sherry, J. Phys. Chem. 70 (1966), 1158), for example, the lithium ion exchange of the X-zeolite is difficult. Where an X-zeolite containing lithium ions in a proportion of not smaller than 80% is prepared by exchanging sodium ions of the Na+-LSX with lithium ions, lithium is required in an amount at least 4 to 15 times the amount of the lithium ions taken into the zeolite.
Literature 2 states that the lithium ions should be contained in a proportion of 88% or more with a SiO2/Al2O3 ratio of 2.0 to 2.5 to ensure a high nitrogen adsorption capacity. Further, Literature 3 states that, where the Na+-LSX is ion-exchanged into the Li+-LSX, the nitrogen adsorption capacity is kept virtually unchanged until the content of the lithium ions reaches about 67%, but the nitrogen adsorption capacity is remarkably improved after the content of the lithium ions exceeds 67%.
To provide an adsorbent having a high nitrogen selective adsorption capacity, the ion exchange with lithium ions intrinsically having a lower exchange efficiency should be caused to proceed to a higher extent and, therefore, a great amount of the expensive lithium source is consumed, resulting in higher costs. For practical application to the oxygen production PSA, it is essential to improve the lithium ion exchange efficiency and to reduce the lithium ion association ratio while maintaining a high adsorption capacity.
In view of the foregoing, it is an object of the present invention to provide a nitrogen selective adsorbent having a high nitrogen selective adsorption capacity even with a lower lithium ion exchange ratio than the conventional adsorbent, and to provide an air separation method.